You don't give us much info such as a URL or anything. I'm going to *guess* that your child links are in a nested UL tag. However, you define links (A tag) with a top and bottom padding of 10px. Since you haven't defined anything different for the A tags in the nested UL they will inherit that properties of the A tags that you have defined.
Do a Google search on CSS to understand how cascading works. Also, you might want to try something like the 'Web Developer' addon for FF so that you can see information such as where a particular element falls in your CSS heirarchy and it also allows you to alter the CSS and see the results immediately.
It's probably something like this:
.menu UL UL a:link, .menu a:visited, .menu a:active {
padding: 4px 0px;
}